1. Field of the Invention
The invention concerns a method for operation of a magnetic resonance system of the type wherein, in which a B1 field distribution of the radio-frequency pulses (“RF pulses”) radiated into an examination volume by a radio-frequency antenna of the magnetic resonance examination system is determined, and then the RF pulses emitted by the radio-frequency antenna are optimized, based on the determined B1 field distribution, for homogenization in a specific volume. Moreover, the invention concerns a magnetic resonance system of the type having a radio-frequency antenna for emission of RF pulses into an examination volume, with a measuring unit to measure a B1 field distribution of the RF pulses radiated into the examination volume by the radio-frequency antenna, and with a control device which, based on the determined B1 field distribution, optimizes the RF pulses emitted by the radio-frequency antenna for homogenization in a specific volume.
2. Description of the Prior Art
Magnetic resonance tomography (MR tomography) has become a widespread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this modality, the body or body part of the patient is initially exposed to an optimally homogenous static basic magnetic field (generally designated as a B0 field) that is generated by a basic field magnet of the magnetic resonance measurement device. During the acquisition of the magnetic resonance images, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding. Moreover, with a radio-frequency antenna, RF pulses of a defined field strength are radiated into the examination volume in which the examination subject is located. The pulse-shaped radio-frequency field that is generated thereby is generally called a B1 field. By means of these RF pulses, the nuclear spins of the atoms in the examination subject are excited such that they are moved from their state of equilibrium, which runs parallel to the basic magnetic field B0, by what is known as an “excitation flip angle” (or “flip angle”). The nuclear spin then processes in the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency receiving antennas. The receiving antennas can be either the same antennas with which the RF pulses were radiated or separate receiving antennas. The magnetic resonance images of the examination subject are ultimately generated based on the received magnetic resonance signals. Every image point in the magnetic resonance image is associated with a small body volume, what is known as a “voxel”, and every brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The association between a resonant radiated RF pulse with the field strength B1 and the flip angle α achieved therewith is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                                                  ⁢                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic ratio (which can be viewed as a fixed material constant for most of the nuclear magnetic resonance examinations) and T is the effective duration of the RF pulse. The flip angle achieved by an emitted RF pulse, and thus the strength of the magnetic resonance signal, consequently depends on (aside from the duration of the RF pulse) the strength of the radiated B1 field. Spatial fluctuations in the field strength of the excited B1 field therefore lead to unwanted variations in the received magnetic resonance signal that can adulterate the measurement result.
For high magnetic field strengths—that are inevitable given due necessary magnetic basic field B0 in a magnetic resonance tomography scanner—the RF pulses disadvantageously exhibit an inhomogeneous penetration behavior in conductive and dielectric media such as, for example, tissue. This leads to the B1 field significantly varying within the measurement volume.
In particular in examinations known as ultra-intense field magnetic resonance examinations, in which modern magnetic resonance systems are used with a basic magnetic field of three Tesla or more, special measures must be taken in order to achieve an optimally homogenous distribution of the transmitted RF field of the radio-frequency antenna in the entire volume.
In United States Application Publication 2003/0184293, the function and an application of a multi-channel transmission array is specified for this purpose. The radio-frequency signal emitted by a radio-frequency transmission amplifier is apportioned via an output splitter and a phase shifter among the individual segments of the array. In this document, however, it is only very generally mentioned that a field homogenization can be achieved with this technique.
A further promising approach for this purpose is specified in German OS 101 24 465, corresponding to United States Application Publication 2004/0155656. In this document, a transmission and reception coil for MR apparatuses is specified that has a number of individual antenna elements (resonator segments) that are arranged around the examination volume within a gradient tube. These antenna elements are interconnected into a large-area volume antenna similar to what is known as a birdcage antenna. The individual antenna elements are electromagnetically decoupled from one another via interconnected capacitors. A separate transmission channel via which the radio frequency feed ensues is associated with each antenna element. Phase and amplitude thereby can be individually predetermined for each antenna element. In principle, this enables a complete control of the radio-frequency field distribution in the examination volume (known as “RF shimming”). It is proposed to improve the homogeneity of the RF field in the entire examination volume in this manner. Since, however, in a magnetic scan, every RF pulse acts in general in a different manner both with regard to its function and with regard to the relevant volumes, this optimization strategy is too restrictive.